This invention relates to a magneto-optic information-carrying medium that enables direct overwriting of new information and old information.
FIGS. 3A and 3B illustrate the principle of the prior art of magneto-optic information reading and writing as shown in "Single Beam Over Write Method Using Multilayered Magneto Optical Media", 28p-ZL-3, Extended Abstracts (The 34th Spring Meeting, 1987); The Japan Society of Applied Physics and Related Societies. FIG. 3A shows an oblique view; FIG. 3B is cross-sectional view showing the main parts. The apparatus in these drawings comprises a magneto-optic information-carrying medium 1, which in turn comprises a glass or plastic substrate 2 and two ferromagnetic or ferrimagnetic layers: a first layer 3 and a second layer 4. The apparatus also comprises an objective lens 5 for focusing a laser beam onto the information-carrying medium 1, where it forms a focused spot 6. Reference numeral 7 designates a region on the first layer 3 where binary data "1" is recorded by magnetization in the upward direction in FIG. 3B. The apparatus also comprises two magnets: an initializing magnet 8 for creating an initial magnetic alignment in the second layer 4; and a bias magnet 9 located opposite the objective lens 5 on the other side of the information-carrying medium 1.
This apparatus operates as follows. A support and drive mechanism not shown in the drawings turns the information-carrying medium 1 in a fixed direction (indicated by arrow a in the drawing). The first layer 3, which is positioned on the incident side of the laser beam, has properties similar to those of the recording layer of the information-carrying media used in ordinary magneto-optic discs, and also operates as a recording layer in the apparatus under discussion. The second layer 4, called the supplementary layer, is provided to enable overwriting; that is, to enable new data to be written over old data directly. Let Tc1 and Tc2 be the Curie temperatures of the first layer 3 and second layer 4, and let Hc1 and Hc2 be the coercive forces of first layer 3 and second layer 4, respectively. These parameters of the two layers satisfy the following relationships:
Tc1&lt;Tc2 PA1 Hc1&gt;Hc2 PA1 Hini&lt;Hc1 PA1 Hini&gt;Hc2
Let us first consider the reading of information recorded on the recording layer, that is, the first layer 3. The objective lens 5 is driven by a driving mechanism not shown in the drawings in the direction of its optic axis and in a direction perpendicular to its optic axis (the radial direction of the information-carrying medium) so as to keep the spot 6 in focus and on the track on the recording layer.
As shown in FIG. 3B, the recording layer is magnetized parallel to its direction. The magnetic alignment is either up or down in the drawing, these two directions corresponding to "0" and "1" of binary codes. Magnetization in the up direction in the drawing represents "1. 38 When the information is read, the spot 6 is focused onto the first layer 3. The magnetic alignment of the first layer is converted to optical information through a well-known magneto-optic effect (such as the Kerr effect); thus the information on the information-carrying medium 1 is detected. The intensity of the laser beam on the information-carrying medium 1 in this reading operation is equivalent to intensity a in FIG. 4. At this intensity the temperatures of regions of the first layer 3 and second layer 4 illuminated by the focused spot 6 do not reach their Curie temperatures Tc1 and Tc2, so the focused spot 6 do not erase the recorded information, that is, the magnetic alignment.
Information is overwritten as follows. The information-carrying medium 1 is subject to an external magnetic field Hini (indicated by arrow b in the drawing) generated by initializing magnet 8 in FIG. 3B. The external magnetic field is related to the coercive forces of the first layer 3 and second layer 4 as follows:
As the information-carrying medium 1 rotates in direction a in FIG. 3B, when the second layer 4 passes over the initializing magnet 8 it is uniformly magnetized in the up direction, regardless of the magnetic alignment of the first layers 3. At room temperature the first layer 3 retains its existing magnetic alignment, being unaffected by the magnetic field generated by the initializing magnet 8 or the magnetic field generated by the magnetization of the second layer 4.
To write the information "1," that is, to magnetize the first layer 3 in the up direction, the laser beam is modulated to an intensity equivalent to b in FIG. 4. The temperature of the region of the first layer 3 where the spot 6 is focused by the objective lens 5 then rises until it exceeds the Curie temperature Tc1 of the first layer 3, destroying the magnetic alignment of the first layer 3. The second layer 4, however, remains below its Curie temperature Tc2, so it retains the upward magnetic alignment given it by the initializing magnet 8. When the region of the first layer 3 illuminated by the focused spot 6 cools, it therefore acquires the upward magnetic alignment in accordance with that of the second layer 4.
To write the information "0," that is to magnetize the first layer 3 in the down direction, the laser beam is modulated to an intensity equivalent to c in FIG. 4. In this case the temperatures in the regions illuminated by the focused spot 6 on the first layer 3 and the second layer 4 rise until they exceed the Curie temperatures Tc1 and Tc2, respectively, causing both layers to lose their magnetic alignment. Due to a weak external magnetic field Hb, with magnetic alignment indicated by arrow c in the drawing, generated by the bias magnet 9 located opposite the objective lens 5 on the other side of the information-carrying medium 1, however, the second layer 4 is remagnetized in the direction of the magnetic field Hb, namely the down direction. When the first layer 3 cools, it acquires the downward magnetic alignment of the second layer 4. In this way the first layer 3 is magnetized in the downward direction. The intensity of the external bias field Hb is weak but within the range consistent with the above operation.
As described above, new information can be overwritten directly on old information by modulating the laser beam between intensities b and c in FIG. 4 in accordance with the binary codes "1" and "0".
When the magneto-optic information-carrying medium is structured as in the prior art described above, however, it is difficult to achieve a simple, compact apparatus configuration because the bias magnet must be placed in alignment with the objective lens on the opposite side of the information-carrying medium.